Normal intervertebral discs between endplates of adjacent vertebrae distribute forces between the vertebrae and cushion vertebral bodies. The spinal discs may be displaced or damaged due to trauma, disease or aging. A herniated or ruptured annulus fibrosis may result in nerve damage, pain, numbness, muscle weakness, and even paralysis. Furthermore, as a result of the normal aging processes, discs dehydrate and harden, thereby reducing the disc space height and producing instability of the spine and decreased mobility. Most surgical corrections of a disc space include a discectomy, which can be followed by restoration of normal disc space height and bony fusion of the adjacent vertebrae or disc replacement between the adjacent vertebrae to maintain the disc space height and motion. Procedures may also involve, either alone or in conjunction with intradiscal implants, positioning implants that extend extradiscally along one or more vertebrae.
Proper alignment and attachment of the implants can be difficult because of variations in the shape and contours of the vertebral body. Poor surface contact between the device and the vertebral body results if the device is mounted over variations in the surface of the vertebral body. While it is not necessary that the entire implant surface contact the vertebral body, better results can be obtained by greater contact. Therefore, it is desirable to shape the surfaces of one or more of the vertebrae to better fit the implant.
Preparation instruments can be guided along a trajectory referenced to the disc space that is determined by bi-secting the angle between the vertebral endplates when the spinal column is in a neutral flexion/extension position. The trajectory reference can also be derived from posterior vertebral body landmarks. While such techniques for establishing the trajectory for guiding preparation instruments have been helpful, there remains room for additional improvements.